This invention is directed to anodes for use in cathodic protection systems.
Cathodic protection of metal structures, or of metal containing structures, in order to inhibit or prevent corrosion of the metal in the structure is well known by use of galvanic or impressed current cathodic protection systems. In impressed current systems, counter electrodes and the metal of the structure are connected to a source of direct current. In operation, the metal of the structure, such as a steel reinforcement for a concrete structure, is cathodically polarized. The steel reinforcement being spaced apart from the anodically polarized electrode becomes cathodically polarized and is inhibited against corrosion. Cathodic protection is well known for metal or metal containing structures such as in the protection of offshore steel drilling platforms, oil wells, fuel pipes submerged beneath the sea, and in the protection of the hulls of ship. A particularly difficult problem is presented by the corrosion of steel reinforcement bars in steel reinforced concrete structures. Most portland cement concrete is porous and allows the passage of oxygen and aqueous electrolytes. Salt solutions, which remain in the concrete as a consequence of the use of calcium chloride to lower the freezing point of uncured concrete or snow or ice melting salt solutions which penetrate the concrete structure from the environment, can accelerate the corrosion of steel reinforcing elements in the concrete. For example, concrete structures which are exposed to ocean water and concrete structures in bridges, parking garages, and roadways which are exposed to water containing salt used for deicing purposes are rapidly weakened as the steel reinforcing elements corrode. This is because such elements when corroded create local pressure on the surrounding concrete structure which brings about cracking and eventual spalling of the concrete.
Known methods of introducing an anode within an existing concrete structure may involve insertion of an anode into a slot cut into the concrete. After application of the anode a cap of grout is applied to backfill the slot. Representative anodes for cathodic protection of steel reinforced concrete structures are disclosed in U.S. Pat. No. 5,062,934 to Mussinelli in which a grid electrode comprised of a plurality of valve metal strips having voids are disclosed. Another type of anode strip for cathodic protection of steel reinforced concrete structures is disclosed in Canadian 2,078,616 to Bushman in which mesh anodes are disclosed consisting of an electrocatalytically coated valve metal which is embedded in a reinforced concrete structure so as to function as the anode in a cathodic protection system. In U.S. Pat. No. 5,031,290 a process is disclosed for the production of an open metal mesh having a coating of an electrocatalytically active material formed by slitting a sheet and stretching the coated sheet to expand the sheet and form an open mesh. In U.S. Pat. No. 4,401,530 to Clere, a three dimensional electrode having substantially coplanar portions, substantially flat portions, and ribbon-like curved portions is disclosed for use as a dimensionally stable anode in the production of chlorine and caustic soda. The ribbon-like portions of the anode are symmetrical and alternate in rows above and below the flat portions of the anode.
In U.S. Pat. No. 3,929,607 to Krause, an anode assembly for an electrolytic cell is disclosed comprising a film-forming metal foraminate structure comprising a plurality of longitudinal members spaced with their longitudinal axis parallel to one another and carrying on at least part of their surface an electrocatalytically active coating. Each longitudinal member comprises a channel blade member constituted by a pair of parallel blades having one or more bridge portions connected to the current lead-in means.
In Canadian 1,325,789 to Martin et al., a cathodic protection system for steel reinforced concrete is disclosed in which a valve metal anode is used. An anode having an electrochemically active surface coating is used. The coating can be an oxide selected from the group consisting of platinum group metal oxides, magnetite, ferrite, and cobalt oxide spinel.
A permeable metal structure useful for acoustical and other special purposes is disclosed in U.S. Pat. No. 3,279,043. The structure is prepared by perforating a metal sheet and bending portions of the metal in a plurality of locations so as to produce what can be characterized as a double corrugated sheet metal. Similarly, in U.S. Pat. No. 3,376,684 a double corrugated sheet is produced from flat sheet metal stock by slitting a metal sheet and bending portions of the sheet so as to provide curved portions above and below the plane of the flat sheet metal stock.
The novel anodes of the invention in comparison with prior art mesh anodes for cathodic protection of steel reinforced concrete provide less resistance to flow of concrete and concrete grout and improved electrical current flow characteristics.
Unitary, multi-plane, porous, anodes in strip form are disclosed which are useful, for instance, deposited within a steel reinforced concrete article as part of a cathodic protection system. Anodes comprising a valve metal, valve metal alloy, or mixtures of valve metals are preferred for use in impressed current cathodic protection systems. For galvanic cathodic protection systems, the metal most often comprises zinc although other sacrificial metals having a higher electropotential than the reinforcing metal of the concrete structure can be used. The following discussion will be restricted to anodes comprising a valve metal for use in an impressed current cathodic protection system. Generally, the anodes of the invention are deposited within a horizontal, inclined, vertical, or overhead surface of said steel reinforced concrete article.
The metal strip anodes of the invention are characterized by a plurality of louvers formed on a first plane of a valve metal strip having the largest area, said louvers defining a second plane or both a second plane and a third plane at the lateral extremities of said louvers. Multiple louver units can be longitudinally spaced apart from adjacent units thus forming a series of multiple louver units. Each unit can be formed of louvers having the same or individually selected angles. Generally, louvers are formed having angles in their long dimension with the longitudinal direction of the metal strip of about 0xc2x0 to about 90xc2x0 and, preferably, 20xc2x0 to about 90xc2x0, and, most preferably, about 70xc2x0 to about 90xc2x0. The louvers form angles with the largest area plane of the metal strip of about 20xc2x0 to about 90xc2x0, preferably, about 70xc2x0 to about 90xc2x0.
The metal strip anodes of the invention bear a plurality of louvers oriented in the long dimension of the louver along the longitudinal or lateral dimension of the metal anode strip. These louvers, preferably, are substantially parallel or substantially perpendicular to the longitudinal dimension of the metal anode strip although, generally, any other orientation of the long direction of the louvers between 0xc2x0 and 90xc2x0 to the longitudinal dimension of the metal anode strip is useful. In addition, when the louvers are oriented in their long dimension along the longitudinal dimension of the metal anode strip, each successive group of louvers, as shown in FIG. 3, can be oriented at any individually selected angle of 0xc2x0 to 90xc2x0 to the longitudinal dimension of the metal anode strip. The louvers define upper and/or lower planes at the extremities of said louvers. Said louvers when oriented parallel to the longitudinal dimension of said strip are bordered at their longitudinal extremities by a plane intermediate between said upper and lower planes. The anode is, generally, formed either on an uncoated valve metal or on a valve metal coated with an electrocatalytically active metal. Said valve metal is selected from the group consisting of titanium, tantalum, zirconium, niobium, and alloys and mixtures thereof. Titanium is the preferred valve metal for forming the anodes of the invention. Where the valve metal is coated with an electrocatalytically active layer, generally, the coating comprises the oxide of a metal selected from the group consisting of a platinum group metal, cobalt, tin, and nickel. It is preferred that the coating comprise a platinum group metal oxide. Where the valve metal anode is uncoated, the usefulness of the anode is limited to cathodic protection systems having anode current densities of up to about 215 milliamps per square meter.